This invention relates to reliability of a product, and in particular to estimating the remaining lifetime of the product, using environmental and use condition history of the product, together with wearout, or aging, acceleration factors applicable to the environmental factors and use conditions.
All products are subject to wearout. Television sets, computers, automobiles, lasers in communication equipment, military equipment, and many other products will eventually fail due to use and environmental conditions in which they are placed. Abnormally severe environmental conditions will accelerate the wearout of the products.
The current owner of such a product is interested in how much remaining life there is left, so the owner can make decisions about when to apply maintenance, whether to upgrade, and when to dispose of the product.
Another party interested in the remaining life of a product is a person considering purchase of a used product. Used computers, for example, often are difficult to sell because potential purchasers have no way of knowing how much the computers have been used, coupled with the environmental conditions in which that use took place.
There are a number of existing methods of providing, to some extent, information that assists either or both of the mentioned interested parties in assessing the age or condition of the product. For example, automobiles have odometers, which displays the distance the vehicle has been driven. The model year of the automobile allows age of the vehicle to be known. Farm tractors display xe2x80x9cengine hoursxe2x80x9d in order to at least inform the interested parties how long the tractor has been operated. Some modern automobiles have sensors in the engine to report the condition of the engine oil, allowing maintenance to be performed when needed.
There are a number of known methods that relate aging, or wearout rates, to environmental use conditions. These methods are widely used in various industries to accelerate failures for purposes of burn-in of product. The semiconductor industry, for example, commonly operates semiconductor chips at an elevated temperature and/or an elevated voltage prior to shipment of the chips in order to weed out xe2x80x9cearly failsxe2x80x9d, also known as xe2x80x9cinfant mortalityxe2x80x9d in the chips. In the burn-in procedure, parts that would have failed within a short time after shipment (weeks or months) are caused to fail at the manufacturer""s site, therefore resulting in a far more reliable product as seen by the customer.
EIA Engineering Bulletin SSB-1.003, xe2x80x9cAcceleration Factorsxe2x80x9d, published by the Electronic Industries Alliance, describes several acceleration factors.
Thermal acceleration effects are accurately modeled by the Arrhenius acceleration factor, which states:
Af=exp(Ea/k*(1/Tuxe2x88x921/Tt)) 
Where
Af=acceleration factor; Ea=activation energy in electron volts (eV); k=Boltzman""s Constant; Tu=use temperature in degrees Kelvin; Tt=test temperature in degrees Kelvin.
As can be seen, for a fixed activation energy, acceleration is quite sensitive to temperature. For example, for metal electromigration on semiconductor chips, an activation energy of 0.6 is typical. If a chip is operated at 100 degrees Centigrade, versus 90 degrees Centigrade, electromigration will be accelerated by approximately a 1.67 factor. Even relatively short times at unusually high temperatures will significantly accelerate thermal wearout mechanisms and therefore detract from the normal life of the product.
Another estimate of an environmental acceleration factor as described in the EIA Engineering Bulletin SSB-1.003 is the Hallberg-Peck acceleration factor that considers humidity effects. Hallberg-Peck includes the Arrhenius factor to account for temperature, but multiplies that acceleration factor by the cube of the ratios of relative humidity, that is:
Af=(RHt/Rhu)3*(Arrhenius acceleration factor) 
Where Af is the Hallberg-Peck acceleration factor; RHt is the test environment relative humidity; RHu is the use environment relative humidity; the Arrhenius acceleration factor is as described earlier. SSB-1.003 goes on to provide further details and variations on the Hallberg-Peck acceleration factor.
SSB-1.003 also describes the Coffin-Manson acceleration factor, which is a common method to model the effects of low-cycle fatigue caused by thermal stressing upon microcircuit and semiconductor package reliability. The Coffin-Manson acceleration factor is:
Af=(xcex94Tt/xcex94Tu)m 
Where Af is the acceleration factor; xcex94Tt is the thermal cycle temperature change in the test environment; xcex94Tu is the thermal cycle temperature change in the use environment; and m is a constant, derived from empirical data. This equation predicts that thermo-mechanical failures are greatly accelerated if the temperatures excursions are larger than expected during thermal cycles.
Many other models exist in the literature that characterize how various environmental conditions affect wearout of products. Complementary Metal Oxide Semiconductor (CMOS) Field Effect Transistor (FET) devices have wearout rates that are highly dependent upon operational voltage applied, and a model sensitive to an exponential of that voltage is used to estimate wearout acceleration caused by unusually high voltage.
It should be noted that although the above models provide estimates for wearout acceleration, actual time of failure at a fixed environmental condition has a significant statistical uncertainty. For example, suppose that, in the electromigration case above, a product might be expected to last between 8 and 12 years at 90 degrees Centigrade. At 100 degrees Centigrade, it would be expected to last between {fraction (8/1.67)} years and {fraction (12/1.67)} years, or 4.8 years to 7.2 years.
Existing products lack a mechanism to provide an indicator of how much of a product""s expected life has been used, incorporating environmental factors and acceleration models utilizing those factors. Neither an estimate of remaining life under such considerations, nor even a simple display of an accumulated acceleration indicator is provided by current art.
Therefore, a need exists to provide an indicator of how much of a product""s expected life has been used, taking into consideration the environmental conditions in which the product has been operated.
A principal object of the present invention is to provide a user or a potential purchaser of a product with an indication of how much of the product""s expected life remains.
In brief, a method and apparatus are provided which senses one or more environmental conditions, inputs those sensed values into one or more age acceleration models, and accumulates outputs of those models over time as a time integration of the age acceleration factors, resulting in an effective age of the product. These accumulations are made available to the user or prospective purchaser in several ways.
In one embodiment, the acceleration factors are simply displayed. In another embodiment, the effective age of the product is displayed normalized to the age of the product in a specified environment. In yet another embodiment, a warning is provided that, with some probability level, the product is likely to fail, or will be likely to fail in the near future, based on how long the product has been used, and under the environmental conditions the product""s use has occurred. In still another embodiment, one or more indicators of a percentage of the product""s expected life that has been used is displayed, considering the environmental conditions in which the product has been used.